Process and apparatus for separating gas mixtures



NW. 1957 E. E. LOCKLAIR ETAL 2,812,645

PROCESS AND APPARATUS FOR- SEPARATING GAS MIXTURES Filed Feb. 29. 1956 A INVENTORS EARL E. LOCKLAIR HARRY J. PORTZER ATTORN Y PROCESS AND APPARATUS FOR SEPARATING GAS MIXTURES Earl E. Locklair, Durham, N. C., and Harry J. Portzer,

Indianapolis, Ind., assignors to Union Carbide Corporation, a corporation of New York This invention relates to an air separation cycle for producing gaseous oxygen and more specifically concerns a method and apparatus for fractionating air in a low pressure air separation cycle to producegaseous oxygen in a state of substantial purity with high oxygen recovery and reduced power requirement.

Heretofore there have been two common methods of air separation which have been generally designated as the single stage or column and the twostage or double column cycles. In the single column cycle all of the rectification trays are located in a single column operating at about 6 p. s. i. g. Air is fed to a condenser in the base or reboiler portion of the column at a pressure which is generally suflicient .for total condensation of the air by heat exchange with the oxygen bath of the reboiler, the minimum necessary air pressure being about 51 p. s. i. g. The single column cannot separate the air completely and operates with a relatively low oxygen recovery. In the customary double column cycle an upper or second stage rectifying column operates at about 6 p. s. i. g. and a lower or first stage rectifying column operates at a suificient pressure to condense pure nitrogen by heat exchange with boiling oxygen in the reboiler at the base of the upper column, the required pressure being about 67 p. s. i. g. The major portion of the air supply is fed to this first stage or lower column and is rectified to pure nitrogen vapor for condensation, and an oxygen-enriched liquid for transfer as feeds to the upper column. The double column devices can separate the air completely into nitrogen and oxygen and are the preferred types in general commercial use. In some instances a minor portion of the air supply is fed as saturated vapor directly into the upper column, but the higher recovery of oxygen is accompanied by thehigher power cost of compressing the air to at least 67 p. s. i. g.

A principal object of the present invention is to obtain substantially complete separation of oxygen from air with reduced power consumption by combining the lower air supply pressure requirement of the single column cycle with the air separating ability of the double column type of cycle. To achieve such object according to the invention, the pure oxygen bath is 'reboiled by a heat exchange that condenses a fraction of cooled supply air as in the single column cycle, but the pure nitrogen that is needed as a feed for the final stage of rectification is condensed at a pressure below that at which the supply air is condensed, by a heat exchange which evaporates at a lower pressure a portion of the fraction of the supply air which was condensed by heat exchange with the reboiling oxygen. The condensation of nitrogen in heat exchange with boiling pure oxygen which characterizes the customary double column cycle is thus avoided.

Prior attempts to provide a lower pressure cycle which will recover substantially all the oxygen in the air have been characterized by condensing pure nitrogen at the pressure at which the supply air is condensed by using an intermediate purity reflux liquid for the refrigerating, medium. .The cycle ofthe present invention, however,

2,812,645 Patented Nov. 12, I957 is distinguished in that after a first fraction of air is partially liquefied by heat exchange with boiling oxygen, the uncondensed portion thereof is turbine expanded and the turbine exhaust is rectified to form pure nitrogen which is condensed to form the required pure nitrogen liquid.

According to the general features of the present invention, compressed air at a relatively lower pressure than is customary for two-stage cycles, is cooled to nearsaturation temperature and atmospheric impurities such as water and carbon dioxide are removed. This may be accomplished by any well known method, and the specific main heat exchange and impurity removal system described herein is merely illustrative. In the illustrative example, the air is cooled in regenerators which are unbalanced for self-cleaning by a side bleed stream as taught by.M. Frankl in United States Patent 2,002,941. This side bleed air stream may be used for turbine preheat as shown by P- K. Rice in United States Patent 2,619,810. The nearly saturated air leaving the' cold ends of the regenerators and the turbine preheat coil may be given a final impurity removal cleanup by a recirculating sciubber-filter-adsorber system as shown by R. W.

Houvener in United States Patent 2,572,933. The regenerators are cooled by the separation products which are previously warmedto nearly the feed air dew point, by heat exchange with scrubbed air as taught in the above.

mentioned Rice patent. A small fraction of clean liquid air is withdrawn from the scrubber to maintain a constantliquid level therein.

According to the invention a cold clean air vapor stream obtained from any suitable cooling and cleaning means, which according to the example is the air vapor stream after the scrubber treatment, is partially liquefied The turbine exhaust is led to a second separator. The

liquid stream fraction of the first separation is throttled into the second separator together with the small fraction of; clean liquid air taken from the scrubber. The purpose. of this second separator is to provide a vapor feed for a lower column which is as rich in nitrogen as possible, and an oxygen-enriched liquid air fraction that is divided into two parts.

. The lower column is fed below its trays with the nitrogen-enriched vapor from the second separator, is fed intermediate the trays with a nitrogen-enriched liquid formed by totally condensing a portion of the nitrogenenriched vapor from the second separator, and is fed above its trays with a nitrogen reflux liquid which is part of the condensed vapor product of the rectification effected by the lower column.

not returned to the top of the lower column is drawn oif for use as top feed to afinal rectification :stage which is effected by a low pressure upper column. V l The upper column is fed at the top with the surplu portion of nitrogen liquid after the latter has been sub cooled and throttled. At a slightly lower level, this upper column is fed with an oxygen-enriched liquid stream obtained by joining the oxygen-enriched liquid product of the lower column with a second portion of the oxygenenriched liquid air from the second separator. At a still lower level a vapor stream is fed to the column which is, characterized by having an oxygen to nitrogen ratio The feeds to the lower column are rectified therein to form an oxygen-enriched suitable for the point of admission but which contains more argon than exists in the column vapor at this feed point. Considerably lower in the column is a level from which vapor is withdrawn, partially liquefied, and to which the liquid fraction thereof is returned. The vapor fraction remaining after such partial condensation is enriched in argon, and is used to increase the argon content of the next higher vapor feed. The next lower feed to the column is an oxygen-rich vapor which is relatively low in argon when'compared to the amount of argon present in the column at this feed point. The lowest feed is high purity oxygenvapor from the main condenser.

An actual plant requires additional heat exchangers as will be explained in greater detail below. In summarizing the above description the chief characteristics of this. cycle include the following:

(1) The supply air is compressed to only enough pressure for partial liquefaction of cold air by heat exchange with the boiling oxygen from the bottom of the low pressure stage or upper column.

. (2) The unliquefied portion of the supply air stream is expanded in a turbine to an intermediate pressure.

(3) The turbine exhaust vapor is rectified in a lower column at' the intermediate pressure to form high purity nitgrogen vapor which can be condensed and used in part to provide upper column top reflux liquid.

.(4) Thev upper column is fitted with extra feeds, bypasses, and heat exchangers to aid in moving the argon up the column for ultimate discharge with the effiuent or waste nitrogen. This permits the attainment of high oxygen purity at the base of the upper column and reduces the quantity of heat that must be transferred to thisoxygen from the condensing air.

Otherobjects, features and advantages of the present invention'will be readily apparent from the following detailed description of a preferred embodiment thereof taken in conjunction with the accompanying drawing, in which:

Fig. l is a diagrammatic flowsheet of the operating cycle'illustrating principles of the present invention.

As shown in Fig. 1, the low pressure air separating system embodying features of the present invention is provided with air at a relatively low pressure, as for example approximately 3 to 4 atmospheres pressure, by means of a compressor unit 11. To remove moisture and carbon dioxide from the compressed air, the air is cooled'in the usual manner in conventional cold accumulators or regenerators 12, 13, 14 and 1S. Reversing heat exchangers can be used instead of regenerators.

As illustrated, separated column products, comprising gaseous nitrogen and oxygen, are passed through conduits l6 and 17 into cold ends of the regenerators 12 and 14, respectively, andout the warm ends at 18 and 19 in the usual manner to cool and clear the regenerators of frozen deposits of moisture and carbon dioxide while counterflowing compressed air is introduced into the regenerators 13 and 14 by proper operation of the reversing valves 20 to cool and freeze the carbon dioxide and moisture content out of the air. The flows are switched To the opposite regenerators for a subsequent period and by operating the reversing valves 20 in a periodic sequential order, a substantially continuous flow of counterflowing air being cooled, and separated gas products being warmed is assured. The air loss of blowdown may be directed through a blowdown branch connection 22 and discharged into the atmosphere.

To assure self cleansing of the regenerator units 12, 13, 14 and 15, the regenerator flows are unbalanced so that only a major portion of the supply air is completely cooled in the colder parts of the regenerators. By this procedure a minor portion of the air being cooled in each regenerator is extracted at points 23 at a temperature below the freezing point of water but slightly above the freezing point of carbon dioxide. This:side bleed air enters a bleed line 24 and is mixed with a portion of the colder, substantially carbon dioxide-free air leaving the cold ends of the regenerators through conduits 25 and 26 by means of a branch connection 27. The proportion of partially cooled air to be combined with the colder air is so balanced as to produce a gas mixture at a temperature below the freezing point of carbon dioxide at the existing air pressure. For example, assuming a withdrawal of about 4.9 cu. ft. per cu. ft. of cooling air in the regenerator at a temperature of about 94 C., upon mixture with about 19.5 cu. ft. of relatively clean air from the cold end of the regenerators at a temperature close to its saturation temperature (about 176 C.), a composite stream at approximately C. is produced. At this temperature practically all of the carbon dioxide in thebleed-off air is precipitated as snow. A valve 29 in bleed line 24 controls the fiow of partially cooled air so as to vary the mixture temperature within a; reasonable range below the fusion point of carbon dioxide.

As previously described, most of the carbon dioxide in the undiverted air is deposited in the regenerator cold end. Most of the C0 in the side bleed air is precipitated by chilling with colder substantially clean air, and the mixture is subjected to a subsequent clean-up step. For this purpose, the carbon dioxide-entrained air is first introduced into a separating device in the form of a whirling or centrifuge apparatus 30 and the carbon dioxide is conveniently and simply separated from the air and removed in the form of snow.

Because themixtu're of the streams of air in the bleed line 24 andthe connecting line 27 is led directly into separating device 30, practically all of the carbon dioxide precipitation takes place within the confines of the separating device 30. In this way only a small percentage of carbon dioxide precipitates outside the separation device 30, thus reducing to a minimum the possibility of harmful deposit of solid carbon dioxide in the equipment.

The relatively carbon dioxide-free air stream leaving the centrifuge apparatus 30 may possibly contain a small percentage of gaseous carbon dioxide of the order of two or three parts per million. Consequently, further purification of the relatively clean air is carried out in a scrubber circuit. Before being purified, this air is cooled to near-saturation temperature in a turbine preheat coil or heat exchanger 31. Such heat exchanger may comprise an air flow passage 32 surrounded by a pass 33 containing a counterflowing cooling agent of air vapor. The near-saturated, relatively clean air leaving the pass 32 is passed through'conduit 34 leading into a conventional' scrubber unit 36. At the time of entering the scrubber 36, this air joins conduit 35 carrying the major portion of the relatively clean regenerator cooled air from the conduits 25* and 26, which remains after diversion through conduit- 27 of that required to cool the air in the bleed line 24'to the desired mixture temperature. The conduit 35 is controlled by an adjustable valve 37. All the supply air thus enters the scrubber 36.

The scrubberu'nit 36 comprises an elongated container havinga sump or well 38 for holding a bath supply of liquid' air and gas and liquid contact trays 39 above the sump. v The air vapor, close to its saturation temperature, is deliveredinto the bath of liquid air in the scrubber sump; 38'through the inlet line 35 which communicates with the interior of the scrubber at or near its bottom but below the levelfof thescrubber liquid, and clean saturated scrubber vapor, free of carbon dioxide contaminants, leavesthe scrubber 36 through a scrubber vapor line 40.

Suitable means: areprovided for providing the scrubber with make-up liquid; As a means for accomplishing this, the scru'bberuriit 36 is providedwith liquefier coils 41 and 42 containing suitable refrigerated-fluids, preferably gaseouslnitrogen and'product'oxygen gas, respectively. These coils 41 and'4 2 arerespectively connected withthe" conduits 1 6 and 17 for cooling the regenerators 12, 13,

The scrubber liquid in the bottom a theiscrixbbe r s6- is withdrawn through a scrubber liquid line 43 and circu'lated through filters 44 by means of a pump 45 interposed in the scrubber liquid line 43. These filters 44 are usually provided in duplicate so that while one is in use, the other may be replaced or cleaned as by thawing. The filtered liquid is passed through a container 46 having an adsorbent material, such as silica gel, for adsorbing any remaining contaminants from the scrubber liquid which might conceivably impair the functioning of the air separation interchanger equipment. Thereafter, the cleaned liquid air is returned to the scrubber 36 by means of scrubber liquid return line 47 where it may be recycled through the scrubber circuit to perform its washing function on the counterflowing air vapor from-line 35.

The liquefied scrubbed air formed-by the liquefier coils 41 and 42, in excess of that needed for make-up purposes, may be extracted through a liquid air line 48, and, being free of deleterious substances, can be fed directly into the air separation apparatus. A valve 48a is provided to control the excess liquid flow and maintain a desired liquid level in the scrubber.

Thus, only highly purified air vapor and liquefied scrubbed air vapor enter the remainder of the system, contaminated scrubber liquid neverbeing transferred outside the closed scrubber series circuit.

Before the scrubbed vapor is utilized in the separation equipment, it is used as anevaporating agent to gasi fy' the highest purity oxygen liquid product from the final rectification stage of the separation cycle. To this end, the scrubbed air vapor in conduit 40 is passed through a main condenser 49 within an oxygen reboiler chamber 50 where, in heat exchange with liquid oxygen at a low pressure entering the chamber 50 through a conduit 51, part of the air vapor is condensed and flows cocurrently with the remaining vapor from the top of the condenser. 49 downwardly, the heat exchange taking place by virtue of the relatively lower boiling temperature of the liquid oxygen at the lower pressure. The gaseous oxygen thus produced leaves chamber 50 upwardly through a conduit 52, and the remaining liquid oxygen leaves through a conduit 53. This liquid oxygen in the conduit 53 is transferred to a side condenser boiling chamber 54 where, in

further heat exchange with a coil 55 through which passes the mixture of gaseous and condensed air vapor from the bottom of the main condenser 49 which is connected to coil 55 by conduit 56, the product liquid oxygen is completely evaporated and leaves the side condenser chamber 54 through a product oxygen gas outlet line 57. The heat of evaporation for the liquid oxygen is supplied by the gaseous air in the coil 55 condensing cocurrently as it travels downwardly, the liquid and gas air mixture finally emerging through outlet conduit 56a at the bottom of th side condenser coil 55. j After leaving the side condenser 54, the productoxygen gas, which is at a relatively low temperature, is subsequently superheated by connecting it to the liquefier coil 42 in the scrubber 36 and then passing it through the oxygen regenerators14 and 15 in the manner described hereinabove.

According to the present invention, the vaporization of the oxygen column product is carried out in the main and the. side condensers 50 and 54 at the lowest possible pressure to avoid the excessive power requirements usually associated with high pressures. l

- It will be noted that the change in composition of the air brought about by condensation causes the condensing temperature to be lower at the bottom of the main condenser 49. As a consequence, the net effect is a much lower temperature drivingforce at the bottom of condenser, which is a limiting factor in determining the conpreviously explained herein--- eem As the partially condensed air from the main condenser 49 passes through the side condenser coil a further vapor fraction is concurrently condensed by heat exchange with the vaporizing oxygen.

As previously stated, the pressure conditions in the main condenser 49 and chamber 50 largely determine the pressure of the compressed air to be produced by the compressor 11. Thus in the above example, the compressed air would be supplied by compressor 11 at about 3 atmospheres pressure, usually less than 50 p. s. i. g., and the scrubber 36 operates at about the same pressure. In the specific example described, a compressed air supply of 44 p. s. i. g. has been found to give a satisfactory heat balance.

Rectification of the air is brought about in interchanger equipment comprising primarily a firststage or lower column 65 and a second stage or upper column 66, the former being primarily for the purpose of producing reflux liquid for said latter column 66, each column being fitted with customary liquid and gas contact rectifying trays.

The dual phase air mixture is delivered by line 56a to a first separator 58 where the from the liquid air, the air vapor being sent by means of a conduit 59 through .the pass 33 of the exchanger 31 for preheating by the warmer side-bleed air in pass 32 prior to work expansion in a turbine 6.2. The liquid from the separator 58 is joined with the excess scrubber liquid from conduit 48 and the combined stream in conduit 60 is throttled through valve 60ainto another or second separator 61. The turbo-expanded air also enters this separator through turbine discharge conduit 63. The nitrogen-rich vapor from this second separator passes through conduit 67 connecting to the bottom of the lower column 65 which receives only a major portion of this vapor, the remainderbeing split off through a branch conduit 68a which has interposed a heat exchange passage within the lower column secondary condenser 68. Such remainder is completely cocurrently condensed by heat exchange with cocurrently evaporating liquid air, and is then fed by the upper part of conduit 68a into the column 65 at an intermediate point thereof. The oxygenenriched liquid fraction emerges from the second separator 61 in bottom connected conduit 64,. and a minor part of it is joined by the oxygen-enriched liquid product in conduit 79 from the bottom of the lower column 65. Conduits 64 and 79 join in a conduit 80 and the combined stream in conduit 80 at about 16 p. s. i. g. is

directed through subcooler heat exchanger for subcooling by efiluent nitrogen, and is throttled through valve 8011 into a high region of the upper column 66 as a reflux liquid feed at about 2 p. s. i. g. This stream is in some respects analogous to the crude oxygen transfer liquid feed in conventional single and double column cycles and the subcooling step minimizes flash-off from throttling.

As previously indicated, the lower column 65 is fed by nitrogen-rich vapor at or near the bottom, and nitrogen-rich liquid at an intermediate point. A nitrogen reflux liquid feed at the top is .also required, therefore.

introduced to the upper ends ofthewlower: and upper air vapor is separated columns 6S'a'nd 66' through split nitrogen transfer conduits 71 and 72 respectively.

Forrcooling and liquefying the gaseous nitrogen in the reflux condenser 70, a major part of the oxygen-rich liquid air fraction in the conduit 64 is diverted by branch conduit 73 into a boiler chamber 88 surrounding condenser 70 after being throttled through an expansion valve 73a to a pressure below the pressure in the lower column, the throttling pressure being such as to depress the boiling temperature of the liquid fraction a few degrees below the liquefactiontemperature of gaseous nitrogen in the reflux condenser 70.

As a result, gaseous nitrogen at approximately the same pressure as in the lower column is condensed against a cocurrently evaporating mixture of cooler, relatively lower pressure liquid and gaseous oxygen-rich air fraction. A dome vent line 70a in the condenser 70 provides for the escape of non-condensibles into the atmosphere. Cocurrent instead of countercurrent' evaporation is preferably used in the reflux condenser to keep the head pressure low. The partially vaporized air formed by this heat exchange transaction leaves the reflux condenser 70 by way of conduit 74, and enters the separator vessel 75 for separating the liquid and gaseous phases. The liquid portion emerges in conduit 76 connecting to secondary condenser chamber 68 where the relatively higher pressure nitrogen vapor in the line 68a is cocurrently condensed. By this heat exchange such liquid portion undergoes partial cocurrent evaporation. The gas from the separator 75 progresses through a flow line 78 for subsequent rectification in the upper column 66.

The upper column 66 comprises an insulated elongated tower having means for receiving and rectifying the liquid and gaseous feeds of varying compositions and temperatures into their component parts. As illustrated in Fig. 1, the upper column is divided into six sections, a column feed being introduced at each section. In the topmost section, gaseous nitrogen effluent or waste nitrogen resulting from the rectification process in the upper column 66 is removed from the dome of the column via nitrogen gas line 81. The waste nitrogen stream is split into streams 81a and 81b, stream 81a flowing through a nitrogen heat exchanger 84, where it subcools the counterflowing, relatively higher pressure nitrogen reflux liquid flowing in coil 82 interposed in the conduit 72. Stream 81b passes through the cold leg of a kettle transfer liquid heat exchanger 85, which subcools the kettle transfer liquid in a coil 83 interposed in the conduit 80. The stream 81a is recombined with stream 8115 by a connection 84a from exchanger 84 to the warmer leg of exchanger 85 and the combined stream passes through the warmer leg of the kettle transfer heat exchanger 85 for further subcooling of the kettle transfer liquid. Finally, the waste nitrogen in stream 86 from the warm end of exchanger 85 passes through a passage 86a extending through the colder part of an upper column secondary condenser 87 and thence to the nitrogen liquefier coil 41, after which it leaves the system via the regenerators 12 and 1.3. The subcooled nitrogen reflux liquid from the nitrogen heat exchanger coil 82 is expanded past throttle valve 72a and delivered into the upper column 66 through nitrogen reflux inlet 89.

At a point some trays below the nitrogen reflux liquid inlet 89, the subcooled kettle transfer oxygen-enriched liquid air from'the kettle transfer heat exchanger coil 83 is throttled through expansion valve 80a and introduced into the upper column 66.

The vapor in the line 78 from the separator 75 is fed directly into the upper column 66 at an intermediate section thereof below the kettle transfer liquid inlet 80a.

Below the vapor inlet line 78, vapor is extracted from the inside of the upper column 66 through an extraction conduit 90 and passed through a warm-to-cold passage 91 extendingcompletely through theupper column second ary condenser 87 where it is partially cocurrently lique- 8 fied. The liquid and gas phases are separated in a vessel 91a, the oxygen-enriched and argon-deleted liquid extract being recycled into the upper column 66 as by means of line 92 at approximately the same level as the extraction conduit 90. The nitrogen and argon-enriched vapor from the separator 91a is delivered to the line 78 by means of a branch connection 93 so that the combined vapor stream enters the low pressure column at an intermediate point above the vapor takeoff 90 and below the kettle transfer liquid feed a. The basir purpose of the previously described partial condensation of column vapor is to transfer a substantial portion of the argon from the lower part to the upper part of the upper column without substantially changing the oxygen content at any point in the column. By so doing, high purity liquid oxygen can be attained at the bottom of this column, and the quantity of heat to be transferred to this boiling oxygen from condensing air is minimized.

The partially evaporated liquid air from the lower column secondary condenser 68 passes through conduit 94 to a passage 95 through the warmer part of the upper column secondary condenser 87 where it is completely cocurrently evaporated. This vapor enters a lower region of the upper column 66 substantially below the conduit through an inlet 96.

The lowest section of the upper column 66 receives the gaseous oxygen through conduit 52 from the main con-' denser boiling chamber 50, and dispenses liquid oxygen separation product through the outlet conduit 51 to the chamber 50 as previously described.

The pressures maintained in the upper column 66 are relatively low, being governed largely by the withdrawal pressure of the liquid oxygen in the conduit 51 and the entering gaseous oxygen pressure in the conduit 52.. Consequently, the pressures in the lower regions of the upper column approximate that of the oxygen in the main condenser 50, and ,the pressures in the upper areas of the upper column 66 are of slightly lower order because of the effects of the rectification process operating therein. The pressure at the top of the column must be sufliciently higher than the efiluent nitrogen outlet pressure at conduit 18 to effect circulation of the nitrogen through its flow paths at adequate velocity.

Using the above-described apparatus, 99 percent gaseous oxygen may be obtained from the side condenser without employment of the 5 atmosphere air head pressure usually associated with gaseous oxygen plants. It is to be understood that when regenerators or passage exchanging heat exchangers are used for warming the product oxygen, such oxygen product is slightly contaminated by water and carbon dioxide and by nitrogen of residual air of reversing operation for cooling and cleaning the regenerators, so that the end product purity may be 98.5% oxygen. If an uncontaminated oxygen product is required, the product oxygen is not passed through regenerators but may be heated in embedded coils in the air-nitrogen regenerators or in a non-reversing pass in a reversing heat exchanger plant.

From the above description it is believed apparent that the system, of this invention constitutes an eflicient separation cycle for producing oxygen with a lower air head pressure and smaller power costs than are usually associated with gaseous oxygen production plants. This improvement is obtained by:

(1) Compressing the supply air to only enough pressure for partial liquefaction by heat exchange with the boiling oxygen of product purity from the bottom of the upper column;

(2) Separation of, warming, and turbine expansion of the unliquefied portion of the supply air to an intermediate pressure between the supply pressure and the upper column pressure;

*(3) Rectification of the turbine exhaust vapor and the (liquid .ai'rfr'a'ctions at. the intermediate pressure in a lower column to form high purity nitrogen vapor which in turn is condensed and used in part as upper column top nitrogen refiux liquid and for the lower column;

(4) The use of extra feeds, bypasses, and heat exchangers to aid in transferring the argon from the lower part to the upper part of the upper column for ultimate discharge with the waste nitrogen to permit the attainment of a high purity in the oxygen liquid at the base of the upper column and to reduce the quantity of heat needed to be transferred to the boiling oxygen from the condensing air.

The closer approach to reversible cycle conditions reas top nitrogen reflux liquid sulting in lower power costs has been obtained without sacrifice of operating controllability. When the main air cooling heat exchange is of the regenerator or passage exchanging heat exchanger type there is a need for unbalancing the flows at the cold zones to insure against carbondioxide accumulation. This could be accomplished by passing a diverted portion of air backward through an unbalance coil passage in the regenerators and recooling this diverted air by preheating the turbine air. As illustrated, the unbalance is accomplished by side bleed withdrawal of a portion of air at 23. The carbon dioxide content of the side bleed is frozen by admixture therewith of cold end diverted air in an amount about four times the amount of the side bleed air. This chilling is found to precipitate substantially all of the carbon dioxide so that it can be readily removed by cyclone type separators or filters or a combination thereof illustrated at 30 and the operating life of the heat exchanger 32 is extremely long. This admixture of a relatively large amount of cold end air with the side bleed air provides amixture which is closer to the warm end temperature of the turbine air in passage 33 but which is of large enough volume for very eflicient heat exchange. A further result is that the slight amount of remaining carbon dioxide is spread over a large heatexchanger surface area which results in long service life of the heat exchanger 32 so that it would be unnecessary and uneconomical to install such heat exchanger unit 31 in duplicate. A further reason for the effective carbon dioxide removal is that the turbine inlet air for this cycle need not have as high a temperature because of the substantially larger volume of air available for expansion in the turbine. The warm end temperature for the heat exchanger unit thus is well below the temperature for elfective freezing of carbon dioxide at the air supply pressure.

The low head pressure possible with this invention is attained by condensing only a small amount of the air to make scrubber liquid by coils 41 and 42 and condensing a large fraction of the air by heat exchange with boiling oxygen of product purity in the successive condensers 49 and 55. The stepwise condensation illustrated is preferred because part of the oxygen can be boiled in reboiler chamber 50 to provide upper column vapor at upper column 66 pressure and the product oxygen can be'evaporated in chamber Met a slightly lower pressure and temperature so as to effect closer approaches to reversibility and liquefaction of slightly more air. This combined with the operation of the lower column 65 at an intermediate pressure (about 1 atmosphere higher than upper column pressure) and the passage of an oxygen enriched vapor. into the lower part of the upper column through line 94, reduces the amount of oxygen vapor required to be fed through line 52 to obtain high purity oxygen product.

A second reason for the low head pressure is the provision of the separate nitrogen liquefying condenser 70 that receives nitrogen of the lower column and the refrigeration of the same by the liquid air fraction (separated by second separator 61 after a first throttle expansion through valve 60:: to intermediate pressure, and passed to the chamber 88 through line 73 after a second throttle expansionhthrough valve 73a to the lower column pres-- sure. The nitrogen at intermediatepressure is thus cone so that anitrogen pressure lower than that of the head pressure is effective to provide all the liquid nitrogen needed for top reflux feeds for both columns.

Another reason why low head pressure sufiices is that less oxygen vapor feed through pipe 52 is required because of the provision of the auxiliary condenser system 87 for effectively reducing the argon content in the lower part of the upper column and transferring such argon to upper parts of such column. Thus less condensation need be effected by the oxygen reboiler and the net result is the production of the needed amount of liquid air while leaving an adequate amount of gaseous air fraction to be taken from first separator 58 through line 59 to the expansion turbine 62. This amount of vapor, when warmed in heat exchange 33 and expanded to the intermediate pressure. is found to be sufiicient to provide the low temperature refrigeration requirement. At the same time at least sufficient air vapor is thus available for efficient operation of the lower column to provide the needed amount of nitrogen which when liquefied supplies adequate nitrogen liquid reflux for the upper column.

It is found that in the lower regions of the upper column the equilibrium curve for oxygen-nitrogen-argon includes a region of inflection at high oxygen purities. Since the actual liquid and vapor compositions in the column are in part determined by the ratio of descending liquid to ascending vapor, the liquid-vapor ratio at the bottom of the column must be kept low because of such inflection of the equilibrium. This, due to the argon, is in contrast to an ideal oxygen-nitrogen equilibrium condition which permits high liquid-vapor ratios. Thus above the region of inflection of the equilibrium curve it is found highly desirable to increase the liquid-vapor ratio so that the actual compositions of vapor and liquid may closer approach their equilibrium values.

In this cycle the heat exchange system 87 accomplishes the desired effects by withdrawal at 90 of argon-containing oxygen-rich vapor at a point above the aforesaid in flection zone, subjecting this vapor to a partial condensation (about 70%) in the coil 91, separating the remaining vapor from the liquid in separator 91a, passing the vapor, which contains most of the argon, to a point of the column above the Withdrawal point 90 as by joining it with the vapor feed 78 by connection 93, and returning the liquid, which is very low in argon, back to the column through return connection 92. The liquid returned helps effectively to maintain desired near-equilibrium compositions above the point of introduction which is preferably at about the same level as the withdrawal point 90. One

, effect is to force argon up toward the top of the column;

condensation of column vapor is preferably effected by heat exchange at first with vaporized liquid portion remaining from partial evaporation of the liquid air fraction which was used to condense nitrogen and which is passed through heat exchange passage interposed in conduit 94; and lastly 'by heat exchange with efiluent nitrogen to the desired extent by coil or passage 86a interposed in the effluent nitrogen conduit 86. However, the improvement may be applied to upper columns in other air separation cycles for similar beneficial effects by effecting the partial condensation of column vapor by heat ex change with other suitable fluids of the cycle, for example expansion of some of the crude oxygen to upper column 11 pressure and evaporating the liquid portion condensing vapor.

The effect ofcondensing a large amount of high purity oxygen vapor at low pressure reduces by a like amount the quantity of nitrogen which must be condensed at higher pressures for use as reflux liquid at the top of the upper column. Since the pressures at which the several vapors condense are lower, the irreversibility of the cycle is greatly reduced, thus permitting high purity oxygen at a high yield to be produced at a substantial power saving.

It may be noted that oxygen of highest purity can be obtained as a final product by modifications of the initial air cooling heat exchanger system to avoid the use of oxygen-air regenerators 14 and 15. For example, as is known to those skilled in the art, the product oxygen can be warmed by passage through embedded coils in the nitrogen-air regenerators or when passage exchanging heat exchangers are used the oxygen would be passed through a non-reversing passage.

It may be noted that the description mentions cocurrent evaporation and condensation at various points. While cocurrent heat exchange is preferred it is not essential to the invention, nor is the physical separation of the main and side condensers which could be combined in a single device. Also, certain auxiliary equipment shown in the specific embodiment such as kettle transfer heat exchanger 68, nitrogen heat exchanger 84, and the heat exchange system for cooling and conditioning the air and warming the eflluent nitrogen and oxygen products which are described for this particular cycle heat balance may be modified or changed to different equipment for cycles having a different heat balance. Thus, such equipment is not to be considered limiting parts of the invention and modifications may be effected without departing from its spirit and scope.

What is claimed is:

1. In a cycle for the low temperature separation of air to produce oxygen of substantial purity by partial liquefaction of compressed air and low temperature rectification of liquid and vapor fractions of the air in stagesin cluding a stage of rectification at low pressure wherein an upper effluent comprising mainly nitrogen and a lower product oxygen is produced, the improvements comprising compressing the air to be separated to only that supply pressure at which it, after cooling, will partially condense by heat exchange against boiling liquid product oxygen of the low pressure rectification, effecting such partial condensation of the air, expanding the resultant gas fraction and at least part of the resultant liquid fractionto an intermediate pressure between the supply pressure and said low pressure, efiecting a separation at intermediate pressure of the expanded vapor fraction to produce an intermediate nitrogen product, effecting condensation of the intermediate nitrogen product by heat exchange against a portion of said liquid fraction after expansion of same to said low pressure, and utilizing at least part of such condensed nitrogen for top reflux feed to said low pressure rectification.

2. A cycle for the low temperature separation of air according to claim 1 in which said gas fraction after partial condensation of the air is warmed by heat exchange with a warmer portion of air prior to expansion and the expansion is effected with production of external Work providing low temperature refrigeration for the system.

3. A cycle for the low temperature separation of air according to claim 1 in which said partial condensation of air is effected in successive stages, the first of which is by heat exchange with reboiling liquid oxygen to provide vapor for the low pressure rectification at said low pressure and a subsequent stage is by heat exchange with evaporating product oxygen at slightly lower pressure.

4. A cycle for the low temperature separation of air according to claim 1 in which said liquid fraction is throttle ,expandedto the intermediate pressure and separated into resulting liquid and gaseous phases, the gaseous phase against the i is joined with the expanded vapor fraction for the separation providing the intermediate nitrogen product, and the portion of said liquid fraction used for effecting condensationof the intermediate nitrogen product is taken from said resulting liquid phase.

5. A cycle for the low temperature separation of air according to claim 1 in which said separation at intermediate pressure provides also a crude oxygen product which is fed to a zone of said low pressure rectification below the top zone, and including the steps of withdrawing from a zone of rectification substantially below the crude oxygen feed but above the lowest zone a substantial amount of vapor which contains argon, subjecting the withdrawn vapor to partial condensation by heat exchange with colder fluid of the system, separating and returning the liquid so produced to the rectification, and passing the vapor remainder of said partial condensation to a level of said rectification above the point of withdrawal of said substantial amount of vapor.

6. A cycle for the low temperature separation of air according to claim 5 in which said colder fluids of the system includes a vapor enriched in oxygen product by evaporation of the liquid remainder of partial evaporation of said portion of said liquid fraction after the heat exchange with the intermediate nitrogen product.

7. In a cycle for the low temperature separation of air to produce oxygen of substantial purity by partial liquefaction of compressed air and low temperature rectification of liquid and vapor fractions of the air in stages including a stage of rectification at low pressure wherein an upper effluent comprising mainly nitrogen and a lower product oxygen is produced, and in which the air is treated at pressures below about p. s. i. g. by steps including heat exchanges and separations producing several liquid and vapor feeds for said low pressure rectification comprising a liquid nitrogen feed to the upper zone of said rectification, a crude oxygen liquid feed to a zone below said upper zone, and a vapor feed of reboiled product oxygen to the lowest zone of said rectification, the improvement comprising withdrawing from a zone of rectification substantially below the crude oxygen feed but above the lowest zone a substantial amount of vapor which contains argon, subjecting the withdrawn vapor to partial condensation by heat exchange with colder fluid of the system, separating and returning the liquid so produced'to the rectification, and passing the vapor remainder of said partial condensation to a level of said rectification above the point of Withdrawal of said sub- .stantial amount of vapor.

8. A cycle for the low temperature separation of air according to claim 7 in which said colder fluids of the system include a fraction in vapor state at 'said low pressure and having higher oxygen content than said crude oxygen such colder fluid after the heat exchange being fed to a zone of the rectification below said point of withdrawal of substantial amount of vapor.

9. A cycle forthe low temperature separation of air according to claim 7 in which said colder fluid of the system includes at least a portion of said upper eflluent comprising nitrogen.

10. A cycle for the low temperature separation of air according to claim 7 in which said vapor feed of reboiled product oxygen is produced by heat exchange of such product oxygen with cold air at the supply pressure for partial condensation of a portion thereof. i

11. In a cycle for the low temperature separation of air to produceoxygen of substantial purity by partial liquefaction of compressed air and low temperature rectification of liquid and vapor fractions in at least a stage of rectification at low pressure providing an upper eflluent of mainly nitrogen and a bottom product of said oxygen 7 and in which air at a supply pressure below 70 p. s. i. g. is cooled to low temperature by heat exchange with at least the upper effluent, the steps comprising effecting partial condensation of the cooled air at the supply pressure by heat exchange against boiling liquid oxygen bottom diate nitrogen product and oxygen-enriched liquid por-- tions; effecting condensation of said intermediate nitrogen product at intermediate pressure by heat exchange with at least part of said oxygen-enriched portions after expansion of same to about said low pressure; utilizing part of the condensed intermediate nitrogen fortop reflux feed to said rectification at intermediate pressure and the balance for top reflux feed to said low pressure rectification; and utilizing the remainder of said oxygen-enriched liquid portions and the part thereof heat exchanged with said intermediate nitrogen product as feeds to said low pressure rectification. q

12. In a cycle for the low temperature separation of air to produce oxygen of substantial purity by partial liquefaction of compressed air and low temperature rectification of liquid and vapor fractions in at least a stage of rectification at low pressure providing an upper eflluent of mainly nitrogen and a bottom product of said oxygen and in which air at a supply pressure below 70 p. s. i. g. is cooled to low temperature by heat exchange with at least the upper effluent, the steps comprising effecting partial condensation of the cooled air at the supply pressure by heat exchange against boiling liquid oxygen bottom product; separating the resultant liquid and vapor fractions of the air; expanding said vapor fraction to an intermediate pressure which is between the supply pressure and said low pressure; expanding said liquid fraction to said intermediate pressure; effecting a second separation by separating the liquid which is enriched in oxygen from the vapor flashed off during expansion of said liquid fraction; passing the separated flash-01f vapor and the expanded vapor fraction to an intermediate pressure rectification for separation therein providing a top intermediate nitrogen product and a bottom crude oxygen product; condensing said intermediate nitrogen product by heat exchange with at least part of the liquid enriched in oxygen from said second separation after expansion to about said low pressure; using liquefied intermediate nitrogen product in part for feed to the top of the intermediate pressure rectification and in part for top feed to the low pressure rectification; passing said crude oxygen product and a remainder of the liquid enriched in oxygen from said second separation to said low pressure rectification at a zone below the top feed zone; feeding the vaporized portion of the liquid enriched in oxygen employed for condensing intermediate nitrogen product to a zone of said low pressure rectification below the feed of said crude oxygen product; and feeding the unvaporized remainder of the liquid enriched in oxygen employed for condensing intermediate nitrogen product to a zone of said low pressure rectification next above the lowest zone.

13. A cycle for the low temperature separation of air according to claim 12 in which an argon containing vapor is withdrawn from said low pressure rectification at a zone above the lowest zone, is fractionally condensed by heat exchange with colder fluids of the system, is separated into liquid and gaseous remainder fractions, the liquid fraction is returned to the same zone, and the gaseous remainder is fed to the low temperature rectification at a higher zone.

14. A cycle for the low temperature separation of air according to claim 11 in which prior to expansion, said vapor fraction of the air is warmed by a heat exchange including cooling at least a part of the supply air and said expansion is with the production of external work.

15. In a cycle for the low temperature separation of air to product oxygen of substantial purity by partial liquefaction of compressed air and low temperature rectification of liquid and vapor fractions in at least a stage of rectification at low pressure providing an upper eflluent of mainly nitrogen and a bottom product of said oxygen and in which air at a supply pressure below p. s. i. g. is cooled to low temperature by passage through reversing heat exchange devices cooled and purged by at least the upper effluent, said devices being unbalanced by side bleed withdrawal of a portion of the air, the steps comprising eifecting partial condensation of the cooled air at the supply pressure by heat exchange against boiling liquid oxygen bottom product; separating the resultant liquid and vapor fractions of the air; preheating said vapor fraction by heat exchange with at least part of said side bleed portion of air; expanding said vapor fraction to an intermediate pressure which is between the supply pressure and said-low pressure; expanding said liquid fraction to said intermediate pressure; effecting a separation including rectification at intermediate pressure of said expanded vapor fraction and said liquid fraction to provide an intermediate nitrogen product and oxygen-enriched liquid portions; effecting condensation of said intermediate nitrogen product at intermediate pressure by heat exchange with at least part of said oxygen-enriched liquid portions after expansion of same to about said low pressure; utilizing part of the condensed intermediate nitrogen for top reflux feed to said rectification at intermediate pressure and the balance for top reflux feed to said low pressure rectification; and utilizing the remainder of said oxygen-enriched liquid portions and the part thereof heat exchanged with said intermediate nitrogen product as feeds to said low pressure rectification.

16. In a cycle for the low temperature separation of air to produce oxygen of substantial purity by partial liquefaction of compresesd air and low temperature rectification of liquid and vapor fractions in at least a stage of rectification at low pressure providing an upper effiuent of mainly nitrogen and a bottom product of said oxygen and in which air at a supply pressure below p. s. i. g. is cooled to low temperature by passing through reversing heat exchange devices cooled and purged by at least the upper efliuent, said devices being unbalanced by side bleed withdrawal of a portion of the air, the steps comprising mixing With said side bleed portion a portion of colder air diverted from the air subsequent to its passage through the reversing heat exchange devices sufiicient to reduce the temperature of the resulting mixture for solidification of substantial amounts of the carbon dioxide impurity in said side bleed air portion; treating the resulting mixture for removal of at least its solidified carbon dioxide impurity content; effecting partial condensation of the cooled air leaving a vapor remainder; preheating such vapor remainder by heat exchange with at least part of the treated side bleed mixture; expanding the preheated vapor remainder with production of external work; and passing the expanded remainder, and the liquid fractions of partial condensation including the treated side bleed mixture to the low temperature rectification for separation.

17. In a system for the low temperature separation of air including means for cooling and cleaning air at a supply pressure below 75 p. s. i. g. in heat exchange with at least the nitrogen effluent product in preparation for treatment in a low pressure rectifying column having an oxygen reboiler associated with the bottom thereof the combination therewith of means for feeding cooled air to the condenser side of said oxygen reboiler for partial liquefaction of the air; means for separating and expanding the vapor remainder to an intermediate pressure between the supply pressure and said low pressure; an intermediate pressure rectifying column connected to receive the expanded vapor; means for expanding the liquid fraction of said partial liquefaction to the intermediate pressure and passing the vapor portion to said intermediate pressure column; an intermediate product nitrogen condenser associated with the upper end of said intermediate pressure column;. means for refrigerating said intermediate nitrogen condenser by. oxygen-enriched liquid expanded to about said low pressure; means for passing part of condensed nitrogen from the intermediate nitrogen condenser to the upper part of the intermediate pressure column and the remainder to the upper part of the low pressure column; and means for passing crude oxygen from the lower end of the intermediate pressure column to said low pressure column at a zone below the top.

18. A system for the low temperature separation of air according to claim 17 including heat exchanger means for preheating said vapor remainder prior to expansion with external work by a heat exchange with at least part of the supply air.

19. A system for the low temperature separation of air according to claim 17 including a second separator connected to receive said expanded liquid fraction and the expanded vapor remainder, and connected to the intermediate pressure column to pass separated vapor and expanded vapor thereto and connected by circuit means to pass at least a portion of separated liquid enriched in oxygen to said means for refrigerating said intermediate nitrogen condenser.

20. A system for the low temperature separation of air according to claim 17 which includes a side condenser having a boiling side connected for receiving liquid product oxygen from said reboiler and a condensing side connected to receive air from the condenser side of said res 16 boiler for condensing additional fractions of the air to form the vapor remainder and the liquid fraction.

21. A system for the low temperature separation of air according to claim 17 which includes means for feeding resulting gas material of the oxygen-enriched liquid used for refrigerating said intermediate nitrogen condenser to said low pressure column; means for withdrawing from a lower level of said low pressure column a vapor containing argon, and fractionally condensing same in a heat exchanger cooled by colder fluids of the system; means for separating and returning the liquid so condensed to the low pressure column at about the withdrawal level; and means for passing the remaining vapor to a higher zone of said column.

References Cited in the file of this patent UNITED STATES PATENTS 2,386,297 Dennis Oct. 9, 1945 2,423,543 Yendall July 8, 1947 2,433,508 Dennis Dec. 30, 1947 2,547,177 Simpson Apr. 3, 1951 2,626,510 Schilling Ian. 27, 1953 2,655,796 Rice Oct. 20, 1953 2,664,718 Rice Jan. 5, 1954 2,664,719 Rice Jan-5, 1954 2,753,698 Jakob July 10, 1956 2,762,208 Dennis Sept. 11, 1956 

1. IN A CYCLE FOR THE LOW TEMPERATURE SEPARATION OF AIR TO PRODUCE OXYGEN OF SUBSTANTIAL PURITY BY PARTIAL LIQUEFRACTION OF COMPRESSED AIR AND LOW TEMPERATURE RECTIFICATION OF LIQUID AND VAPOR FRACTIONS OF THE AIR IN STAGES INCLUDING A STAGE OF RECTIFICATION AT LOW PRESSURE WHEREIN AN UPPER EFFUENT COMPRISING MAINLY NITROGEN AND A LOWER PRODUCT OXYGEN IS PRODUCED, THE IMPROVEMENTS COMPRISING COMPRESSING THE AIR TO BE SEPARATED TO ONLY THAT SUPPLY PRESSURE AT WHICH IT, AFTER COOLING, WILL PARTIALLY CONDENSE BY HEAT EXCHANGE AGAINST BOILING LIQUID PRODUCT OXYGEN OF THE LOW PRESSURE RECTIFICATION, EFFECTING SUCH PARTIAL CONDENSATION OF THE AIR, EXPANDING THE RESULTANT GAS FRACTION AND AT LEAST PART OF THE RESULTANT LIQUID FRACTION TO AN INTERMEDIATE PRESSURE BETWEEN THE SUPPLY PRESSURE AND SAID LOW PRESSURE, EFFECTING A SEPARATION AT INTERMEDIATE PRESSURE OF THE EXPANDED VAPOR FRACTION TO PRODUCE AN INTERMEDIATE NITROGEN PRODUCT, EFFECTING CONDENSATION OF THE INTERMEDIATE NITROGEN PRODUCT BY HEAT EXCHANGE AGAINST A PORTION OF SAID LIQUID FRACTION AFTER EXPANSION OF SAME TO SAID LOW PRESSURE, AND UTILIZING AT LEAST PART OF SUCH CONDENSED NITROGEN FOR TOP REFLUX FEED TO SAID LOW PRESSURE RECTIFICATION. 